"dwindling interest" is grammatically correct and can be used in written English.
You can use it to describe a situation in which interest or enthusiasm in something is gradually decreasing over time. For example: "With the recent news surrounding the company, the public's dwindling interest in investing has been evident."
The league limped along with dwindling interest for at least two seasons after play resumed.
Sadly, within sport, there now appears to be a dwindling interest in staging the Games.
The bizarre winter storm also carried the threat of below-freezing temperatures, with the potential for wholesale leaf loss quickly followed by dwindling interest in this series.
Despite my dwindling interest in parenting manuals, I couldn't resist a new release with a title that I felt encapsulated my child-rearing philosophy.
As more and more women squeal louder and louder, my dwindling interest in watching these great players will devolve to no interest at all".
And the call for universal background checks for gun ownership is increasingly in doubt as Senate Republicans, sensing dwindling interest, resort to usual vows to filibuster.
Worst of all, they fail to enjoy spying as a treacherous game; they think they are saving the world, whereas the Brits know that, apart from Britain's dwindling interests, there's nothing to be saved, just the endless struggle itself, well or poorly joined.
Indeed, Concorde was only certified late in 1975 after years of testing, dramatic cost overruns and dwindling customer interest.
Some Democrats outside Washington said they were most disturbed that a dwindling public interest in politics had filtered into their own increasingly depleted ranks.
Minnawi praised the efforts of Hollywood actor George Clooney to keep the plight of Darfur in the spotlight despite dwindling media interest.
Jones turned out to be the pen name of Margaret (Peggy) Seltzer, a prep-school graduate who, perhaps noticing the dwindling public interest in literary fiction, rebranded her novel as a memoir.
